Astrodon Narrowband 3nm H-Alpha and 3nm OIII Filters
Submitted: Monday, 20th April 2015 by Slawomir Lipinski
There are many things that interfere with successful astro-imaging. Some obstacles can be overcome, while factors such as overcast or strong wind can provide with opportunities for catching up on sleep. Since I started my journey in astrophotography just a few years ago, I had to deal with relatively strong light pollution.
Until now a set of 12 nm narrowband filters has been very useful and provided me with valuable learning opportunities. A move closer to Brisbane’s CBD has been catastrophic for my astro-images; heavy light pollution made it very difficult to capture anything but the brightest DSOs.
Nevertheless, every difficulty gives a valuable opportunity to acquire experience, thus I started searching for ways to overcome dreaded sky glow.
Long story short, after doing quite extensive online research, I eventually decided to acquire Astrodon’s 3 nm Halpha filter and was thrilled with its capacity to block unwanted signal while allowing precious photons that travelled for tens of thousands of years pass through the telescope and be converted into electrons on the CCD.
With the aim of moving away from blending channels and to be able to start composing vivid symphonies of vibrant cosmic light, i.e. true HOS images, Astrodon’s 3 nm OIII filter has been purchased recently and will be eventually followed up by a 3 nm SII filter.
Although I could perceive significant differences between the performance of my 12 nm filters and new Astrodon’s 3 nm filters, I decided to make a more thorough comparison of these filters. A single 180-second frame of IC 2944 has been captured through various narrowband filters with QSI 690i , binned 2x2, with temperature set at -15 C. All images have been captured on the same night, one after another within minutes, with the same gear – 102 mm ED doublet at F5.6 on AZ-EQ6.
It is actually somewhat challenging to accurately compare single frames from filters with different bandwidths, as each will have different optimal exposure for given DSO to obtain maximum possible Signal to Noise Ratio. In an attempt to objectively visualise data, raw frames have been used with no stretch applied. In order to generate usable jpg files with Nebulosity, instead of applying a stretch, histogram values were manipulated. Minimum histogram value was set the same as minimum ADU value for the entire image, while maximum histogram value was determined by subtracting minimum ADU value from the Mean, then multiplying that difference by a factor of five (5), and finally adding this calculated value to the mean ADU value. Also the halos around a bright star have been compared for the 12 nm and 3 nm OIII filters.
What has been highlighted by this simple comparison is that 3 nm filters do not need longer exposures than 12 nm filters, and as expected, the degree of recorded detail for equal exposures is higher for 3 nm filters – particularly with the oxygen filter where the signal from DSOs is generally weaker.
Moreover, images through 3 nm filters appear less noisy than those obtained with 12 nm filters. That should be of no surprise, since images obtained with 3 nm filters will suffer not as much from sky glow and it is precisely the sky glow that significantly lowers the SNR by introducing extra shot noise without adding any information from the DSO.
In conclusion, 3 nm filters make longer exposures possible in the areas of heavy light pollution by effectively blocking the sky glow and consequently allow for capturing significantly fainter details comparing with 12 nm filters.
Again, single raw frames were used with no stretch applied. To accommodate for different ranges for ADU values in both images, histogram values were adjusted as per previous method. Comparatively small halos around stars with 3 nm filters should make successful colour composition of narrowband images much easier.
My set of 12 nm filters has been relatively parfocal on 80 mm F6 triplet, however, when used with my 102 mm F5.6 ED doublet, I had to significantly adjust focus between different 12 nm filters and that consequently lead to re-focusing auto guider camera attached to an OAG. For this very reason I went back to using a guide scope for some time, although it also meant wrestling with differential flexure. Thus it was a very pleasant surprise to notice that Astrodon’s filters are truly parfocal even on my 102 mm ED doublet. After changing from Astrodon’s Halpha to Astrodon’s OIII filter on the same refractor, only a cosmetic change of focus was required and the auto guider attached to an OAG remained at prime focus.
Astrodon by creating 3 nm filters has made astrophotography possible even for city dwellers who can now actively contribute to humanity’s growing cosmic database. Astrodon’s 3 nm narrowband filters perfectly compliment my QSI 690i camera providing me with a very powerful tool for astrophotography.
Thank you for reading this review.